In the last years of her life, the great American reporter Martha Gellhorn became aware of a writer trying to muckrake down among her most personal history. For someone so very public yet so very private, the idea was appalling. She told her friend, the journalist John Simpson: "There's some wretched little man who's gone around digging out material about me, and he's just waiting till I die to bring it out." That man was Carl Rollyson. Last week his finished product, Beautiful Exile, The Life of Martha Gellhorn, was published, three years after her death.

The woman who changed the face of war reporting, giving accounts of the suffering of real people, was right to be worried. Rollyson - who in 1990 published a previous edition of the book in the US but was forced to remove certain sections after a legal battle with Gellhorn - writes in his introduction: "She knew how important sex was for most men, and she used it not only to please men but to get good stories. She slept with generals; she had one-night stands with ordinary soldiers who might not survive the next day. She dressed elegantly, used make-up skilfully, flirted and coaxed men to do her bidding."

A picture emerges of a woman who "smoked, drank and travelled with abandon"; she is castigated for her beauty, her "wheat coloured hair", for wearing Saks Fifth Avenue slacks while reporting on the horrors of war. She is portrayed as a user of men and is said to have tired of her adopted son - a notion he has fiercely rejected.

In her final years in London, perched in a sixth-floor flat in Cadogan Square, Gellhorn had visits from a host of great war reporters and other journalists who had become friends. Most are disgusted by the book.

As the journalist John Pilger says: "The problem Martha Gellhorn still presents to the jealous, envious and scandalmongers is that she was brave, beautiful and clever, and had passionately held political principles. Worse, she was a woman who was decades ahead of her time in pushing the boundaries of her gender. Salacious demolition jobs on remarkable human beings after their death are not new; and they are as craven as ever."

Since she walked out on Ernest Hemingway in 1943, after five years of marriage, Gellhorn had refused to talk much about him. She was a writer in her own right, a woman who had covered the heaviest of wars, and she wished to be remembered for that. Yet all people recalled was the marriage.

Beautiful Exile makes it apparent why Gellhorn was so reticent to talk about the man who would eventually shoot himself. For the first time, her friends have learned that, jealously, he tried to destroy her reputation, dehumanising her as an "operative" during their years working together and trying to turn her into his property.

The book contains extracts from ferocious letters Hemingway wrote to Gellhorn after she walked off to war and out of their marriage. He called her a "career bulldozer", a "product of beauticians" and a "phoney and pretentious bitch".

Rosie Boycott was friends with Gellhorn from the early 70s when Boycott founded Spare Rib and Gellhorn told her the women's movement was "bunk". Boycott helped her find a lawyer to oppose publication of Rollyson's original biography and recalls "quite a nasty fight. I imagine he was monumentally pissed off." She says of the new book: "He comes up with all sorts of things - repeated gossip and rubbish. The stuff about how mean and vitriolic Hemingway was is astounding. I was taken aback by how he set out to destroy her. The only thing I came away with from that book was another level of understanding of why she was so insistent in not wanting to be portrayed as Mrs Ernest Hemingway."

But Gellhorn had seen the viciousness coming. In spite of her friends' pleadings, just before her death she burnt the letters from Hemingway. Following publication of the original biography, Gellhorn wrote 25 pages of notes on Rollyson's errors.

Bill Buford, the New Yorker editor who was a friend of Gellhorn's, explains: "Martha's objections to the prospect of the book were these: the author's previous works were distinguished by their relish for gossip and weak critical acumen and bad writing. She concluded that the man was interested only in salacious gossip. Martha asked many people to do what they could to stop the book."

The essence of Martha Gellhorn was that she was an ambiguity, a woman who was rankled by myth-like people but became a myth herself. In those later years she told stories to her "chaps" - those who came to read to her when she was near-blind following a botched cataract operation - which were hazily recollected through the mist of Famous Grouse. She knew that this way they would be half-remembered and they were rarely written down.

The journalist James Fox, who bonded with Gellhorn when they were writing about the miners' strike, is outraged by the notion that Gellhorn slept with people to get a good story and by the book's claims that she did not actually enjoy sex. He says: "Of course Martha had affairs. Hemingway was allowed to have affairs and that's fabulous and macho. She's portrayed as a scarlet woman. It's as if the author said: 'Hey, let's think of a new angle - Hemingway was a sweetie and Gellhorn was this voracious, vicious, heartless woman'."

Martha Gellhorn was born in St Louis, Missouri, to a mother who was a leading light in the suffrage movement. At 21, she talked her way into a free passage on a liner and sailed to Paris with just $75 in the hope that she could become a foreign correspondent. Four years later, in 1934, she returned to the US and wrote about the depression from the ground, living with the poorest families and railing to President Roosevelt. Her acclaimed book, The Trouble I've Seen, is an account of this time.

She met Hemingway in 1936 when she was on holiday with her family in Key West. It was when the two hooked up in Spain while covering the civil war that they began their affair. She was writing for Collier's magazine.

After the war they lived together in Cuba and travelled to China to cover its war with the Japanese. Back in Cuba, they settled down to domestic life but amid its passion it was a destructive relationship and a stifled Martha felt the pending war had to be written about. Eventually she went off to the front again and when he followed later, Hemingway stole the Collier's commission from under her. In spite of women being barred from reporting from the front line, she one-upped Hemingway by sneaking on to a hospital ship and getting on to the beach during the D-day landings. Hemingway was left to write from a ship. It seems he never forgave her this or the success of her war novel, The Wine of Astonishment.

Gellhorn wrote about the liberation of Italy, Israel, Russia, Poland, Vietnam, she was there when the gates of Dachau were opened after the Nazi surrender. Even in her 80s she was writing about Brazilian street children. She did not listen to "official drivel" and told the truth as she saw it.

Buford says: "Martha was passionate and political, glamorous and exciting. She loved to drink and gossip and smoke and flirt. She was hugely entertaining. She was motivated by a deep-hearted, deep-seated concern for justice; she was the friend of the dispossessed, the oppressed, the neglected. And she was a good writer." And she just happened to be beautiful into the bargain.